1. Field of the Invention
The invention relates to appartus for coating a moving sheet or web with a liquid and more particularly relates to apparatus for creating a predetermined pattern of lines on a moving sheet with a precise volume of liquid.
2. Brief Description of the Prior Art
Recently issued U.S. Pat. No. 3,964,871 describes chemical spot test indicators of the type which comprise a hydrophobic support having disposed thereon a plurality of reagents for the detection and quantification of chemical substances dissolved in biological liquids. The reagents are generally deposited from aqueous solutions.
In most quantitative titrations, it is necessary for accuracy that precise and known quantities of test reagent be exposed for reaction to a fixed volume of solution to be tested. Thus, in chemical spot test indicators of the type described in U.S. Pat. No. 3,964,871 it would be ideal to have a known and specific quantity of reagent deposited on a support member in such a manner that upon immersion in the solution being tested, the reagent remains in place and the site is exposed to a fixed and predetermined volume of solution being tested.
It has been discovered that the highest degrees of accuracy of such test indicators is dependent in part upon the conditions of manufacture. More specifically, the most accurate indicators of the type disclosed in U.S. Pat. No. 3,964,871 are those prepared by applying a constant volume of reagent (in liquid form) to a constant area of the hydrophobic support member. Theoretically the dried reagent sites will be uniform in respect to configuration, dimensions, weight of reagent and location in respect to surrounding hydrophobic surface. If the reagent sites are uniform in regard to the above parameters, they may be expected to imbibe uniform volumes of test specimens. The latter is essential for accurate quantitative analysis by comparative methods of those skilled in the art appreciate.
The art is replete with descriptions of apparatus designed to make uniform coatings on moving webs. For example, U.S. Pat. No. 3,964,871 describes apparatus which serves to apply a coating of uniform thickness to a moving web by applying the coating in excess and then removing the excess. In U.S. Pat. No. 2,887,087 and 3,292,573 disclosure is made of apparatus for applying uniform weights of coating materials over given surface areas of a moving web by altering the concentration of the coating material to compensate for changes in the speed of the moving web. The U.S. Pat. No. 3,186,378 describes apparatus for the accurate deposit of a liquid in a straight line on a moving web, parallel to the web edge. U.S. Pat. No. 3,402,695 discloses apparatus for coating the whole surface of a moving web having varied width, by controlled pumping or supply of the coating to the applicator head. The control is derived from sensing the speed of the moving web.
Although the prior art apparatus has evolved to meet one or more needs also found in the manufacture of chemical test indicators of the type described in the U.S. Pat. No. 3,964,871, no single machine or apparatus has been available to meet all of those needs, i.e.; the need to deposit a constant volume of an aqueous liquid on a constant area of a hydrophobic sheet so as to obtain lines of reagent sites which will imbibe uniformly the same volumes of test specimen at any site where contact with the specimen for analysis is made. The apparatus of the present invention accomplishes this desired result. The apparatus of the invention provides a means to manufacture chemical spot test indicators of the type which comprise a support member and a plurality of reagents adhered thereto, which is useful for the detection and quantitation of a chemical substance dissolved in aqueous media. The apparatus obviates a number of the above-described shortcomings of the prior art devices in that it provides for a precise proportion of test reagent to be applied to a plurality of given sites on the support. The test indicators prepared with the apparatus of the invention will imbibe at each of the reagent sites, a predetermined volume of test solution (the volume which will be predetermined volume of test solution (the volume which will be imbibed is proportional to the volume of aqueous test reagent deposited on the support). Thus, it is possible to control the ratio of reagent to specimen by the accurate control of the volume of reagent liquid deposit applied to the support, relative to the size and configuration of the site receiving the liquid deposit. The test indicators prepared with the apparatus of the invention provide more rapid readout times and improved accuracy. The apparatus of the invention is particularly advantageous for the manufacture of spot test indicators being a plurality of reagent sites for comparative readout.